Absorbent must be permanently supplied to the bed in dependence on the sulfur contents in the fuel. Bed material and ashes formed from the fuel or slag particles which do not accompany the combustion gases must be continuously or intermittently removed to prevent the bed level from rising. Fine-grained material, such as finer fractions of ashes and particles abraded off lumps of bed material, accompany the combustion gases, and are separated in a cleaning plant and transported away therefrom. Bed material, ashes and slag have to be removed by being tapped off from the bed vessel through a controllable discharge device.
Brannstrom et al U.S. patent application Ser. No. 917649 (filed Oct. 10th, 1986) describes a power plant with a fluidized bed, in which the bed level is controlled by grinding down bed material and directly returning the ground down bed material to the bed. The grinding takes place to such a small particle size that the ground bed material leaves the bed vessel together with the combustion gases, is separated in a cleaning plant and is removed, suitably through a pneumatic pressure reducing discharge device for example of the kind described in European Pat. No. 0 108 505 Swedish patent application No. 8602486-6 describes a PFBC power plant with a discharge device for bed material with a slag crusher in which slag lumps above a certain size are crushed in such a dimension that they can be transported to a collecting container through a pneumatic pressure reducing transport device.